Body- or Tip-Controlled Reactivity of Gold Nanorods and Their

Langmuir 2007, 23, 9463-9471

9463

Body- or Tip-Controlled Reactivity of Gold Nanorods and Their Conversion to Particles through Other Anisotropic Structures T. S. Sreeprasad, A. K. Samal, and T. Pradeep* DST Unit on Nanoscience (DST-UNS), Department of Chemistry and Sophisticated Analytical Instrument Facility, Indian Institute of Technology, Madras, Chennai 600 036, India ReceiVed March 27, 2007. In Final Form: May 28, 2007 We report the shape transformation of gold nanorods to spherical nanoparticles, assisted by cupric ions. The reaction proceeds through a series of structures and could be arrested at any stage to produce particles of desired shape. In the presence of a larger concentration of cetyltrimethylammonium bromide (CTAB), selective etching of the tips of the nanorods occurs to a greater extent. The subsequent transformation is driven by the surface reconstruction of nanorods to generate more stable surfaces. As the stability of various surfaces depends on the protecting agent used, the reactivity is modified by controlling its presence at the surface. We show that the body of the rods is more susceptible for reaction at reduced CTAB concentrations. During the conversion to particles, several anisotropic transient structures were observed and were imaged using high-resolution transmission electron microscopy (HRTEM). The transformation occurs due to the hydroxyl radicals produced by Cu2+ in the presence of ascorbic acid (AA). A mechanism has been proposed and several control experiments were conducted to test it. The cupric ion induced shape transformations can be extended to other ions, and knowing the mechanism allows the control of the process to stabilize various anisotropic structures.

Introduction Metals and semiconductors on the nanoscale exhibit properties that can be tailored by varying parameters such as size, shape, and dielectric constant, to mention a few.1 Photophysical properties of metal nanoparticles in the visible and near-infrared region can be varied by tuning their size, shape, aggregation state, and environment.2 Controlling the size and shape of nanoparticles has been a challenge, and usually the size manipulations are done during the synthesis itself. Therefore, recent research is directed toward developing newer synthetic routes to achieve highly monodisperse structures.3 Several groups have used colloidal self-assemblies as templates for controlling the size and shape of nanocrystals.4 In these synthetic protocols, a variety of factors such as ionic concentration and temperature need to be controlled during the synthesis to ensure monodispersity. The surface structure of the nanoparticle is a critical property that determines the reactivity of the particles. However, this is difficult to study with spherical nanoparticles, because of their structural isotropy. Spherical metal nanoparticle surfaces are primarily composed of either {111} or {100}. Usually {110} * Corresponding author. Fax: + 91-44 2257-0545. E-mail: pradeep@ iitm.ac.in. (1) (a) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J. and Edwards, P. P. Chem. Soc. ReV. 2000, 28, 27. (b) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (c) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (2) (a) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (b) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Eur. J. 2002, 8, 30. (c) Brus, L. Appl. Phys. A 1991, A53, 465. (d) Henglein, A. Chem. Rev. 1989, 89, 1861. (d) Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.; Aussenegg, F. R. Opt. Commun. 2003, 220, 137. (3) (a) Hiramatsu, H.; Osterloh, F. E. Chem. Mater. 2004, 16, 2509. (b) Rao, C. N. R.; Agrawal, V. V.; Biswas, K.; Gautam, U. K.; Ghosh, M.; Govindaraj, A.; Kulkarni, G. U.; Kalyanikutty, K. P.; Sardar, K.; Vivekchand, S. R. C. Pure Appl. Chem. 2006, 78, 1619. (c) Hyeon, T.; Chung, Y.; Park, J.; Lee, S. S.; Kim, Y. W.; Park, B. H. J. Phys. Chem. B. 2002, 106, 6831. (d) Yu, H.; Gibbons, P. C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem. Soc. 2001, 123, 9198. (4) (a) Asefa, T.; Lennox, R. B. Chem. Mater. 2005, 17, 2481. (b) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B 1998, 102, 9985. (c) Govindaraj, A.; Satishkumar, B. C.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202. (d) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmaurice, D. Adv. Mater. 2000, 12, 1430.

facets will be absent owing to their inherent instability compared to the other two low index planes.5 Gold nanorods are versatile systems for diverse processes.6 The absorption spectrum of gold nanorods has two characteristic maxima.6 The first one, called the transverse surface plasmon resonance (TSP), is located around 520 nm. The TSP shifts to shorter wavelengths with increasing nanorod aspect ratio. The other maximum, known as the longitudinal surface plasmon (LSP) resonance, appears at a longer wavelength. This band exhibits a red-shift with increase in aspect ratio. Various aspects such as the chemistry,7 one- and two-dimensional assemblies as well as self-assembly on substrates,8 photothermal properties and electron dynamics,9 catalysis,10 and biological applications11 of these systems have been investigated. For any specific application, (5) (a) Todd, B. D.; Lynden-Bell, R. M. Surf. Sci. 1993, 281, 191. (b) Uppenbrink, J.; Johnston, R. L.; Murrell, J. N. Surf. Sci. 1994, 304, 223. (6) (a) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (b) Van der Zande, B. M. I.; Bohmer, M. R.; Fokkink, L. G. J.; Schonenberger, C. J. Phys. Chem. B. 1997, 101, 852. (c) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (d) Juste, J. P.; Santos, I. P.; Liz-Marz’an, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870. (e) Gole, A.; Murphy, C. J. Chem. Mater. 2005, 17, 3668. (f) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (7) (a) Zweifel, D. A.; Wei, A. Chem. Mater. 2005, 17, 4256. (b) Mann, S.; Davis, S. A.; Hall, S. R.; Li, M.; Rhodes, K. H.; Shenton, W.; Vaucher, S.; Zhang, B. J. Chem. Soc., Dalton Trans. 2000, 3753. (c) Kang, S. K.; Chah, S.; Yun, C. Y.; Yi, J. Korean J. Chem. Eng. 2003, 20, 1145. (d) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M.; Lyon, L. A.; Natan, M. J.; Mallouk, T. E. AdV. Mater. 1999, 11, 1021. (8) (a) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (b) Chang, J. Y.; Wu, H.; Chen, H.; Ling, Y. C.; Tan, W. Chem. Commun. 2005, 1092. (c) Jana, N. R. Chem. Commun. 2003, 1950. (d) Jana, N. R.; Gearheart, L. A.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909. (e) Joseph, S. T. S.; Ipe, B. I.; Pramod, P.; Thomas, K. G. J. Phys. Chem. B 2006, 110, 150. (f) Correa-Duarte, M. A.; Liz-Marza´n, L. M. J. Mater. Chem. 2006, 16, 22. (9) (a) Song, D. K.; Lenggoro, I. W.; Hayashi, Y.; Okuyama, K.; Kim, S. S. Langmuir 2005, 21, 10375. (b) Imura, K.; Nagahara, T.; Okamoto, H. J. Phys. Chem. B 2004, 108, 16344. (c) Rex, M.; Hernandez, F. E.; Campiglia, A. D. Anal. Chem. 2006, 78, 445. (d) Sudeep, A. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516. (10) (a) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2005, 44, 744. (b) Song, H. J.; Kim, F.; Kim, D.; Yang, P. Chem. Eur. J 2005, 11, 910. (c) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St. Angelo, S. K.; Cao, Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H. J. Am. Chem. Soc. 2004, 126, 13424.

10.1021/la700851x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007

9464 Langmuir, Vol. 23, No. 18, 2007

size and shape control is vital. Significant research efforts have been made to evolve strategies for synthesizing nanorods of desired aspect ratio.6 An alternate strategy has been used to resize a given ensemble of nanorods, obtained from established procedures, using external stimuli such as thermal/laser heating12 and chemical methods.13 Gold nanorods are different from their spherical counterparts, since their surface is considerably different. In a typical rod sample, the rod axis will be oriented along the [001] direction and the side faces are composed of {110}. The tips will be mainly composed of {111} and {100} facets.14d It is understood that the presence of cetyltrimethylammonium bromide (CTAB), which is used as the capping agent, is responsible for the presence of {110} facets on the nanorod surface. Due to the larger interatomic distance in the {110} facet compared to other low index planes of gold, the bonding between surface gold atoms on the {110} facet will be weaker compared to those on {111} or {100}. This facilitates each atom on the {110} facet to form stronger bonds with adsorbed or bonded atoms. So the {110} surface is preferentially stabilized by CTAB and this is the principal reason for the existence of this on the nanorod. The reconstructions happen where there is a disruption in the CTAB bilayer capping. There exist subtle differences in surface energies of different planes of a nanocrystal that are related to the stability and hence reactivity of these surfaces. This gives a handle to manipulate and control reactions to occur preferentially on specific surfaces, leading to resizing and reshaping of the particles. Due to the higher surface energy of the {110} facet, it is prone to reconstructions. Reconstructions can be brought about by the addition of external stimuli also. Recently, Stucky et al.13b reported a method for the selective shortening of nanorods by oxidation. Selective oxidation happens, resulting in the shortening, as observed in the simultaneous decrease in the intensities of LSP and TSP. Laser irradiation12b can be used for manipulating the aspect ratio of nanorods by melting. But the experimental setup requires sophistication. Thermal reshaping12a of nanorods is another method for postsynthesis manipulation. But for the complete conversion of rods to particles, a comparatively higher temperature (∼160 °C) is required. At 100 °C, the shift observed in the LSP was 60 nm and no reaction was observed further, even after prolonged heating.12b Therefore, in-order to obtain different aspect ratio nanorods, different temperatures are required. In this paper, we report a simple reaction between Au nanorods and cupric ions that induces surface reconstructions and results in a near-quantitative resizing of nanorods. The increase in the intensity of TSP occurs concomitantly with the decrease in LSP, (11) (a) Salem, A. K.; Searson, P. C.; Leong, K. W. Nat. Mat. 2003, 2, 668. (b) Takahashi, H.; Nidome, Y.; Yamada, S. Chem. Commun. 2005, 2247. (c) Chen, C. C.; Lin, Y. P.; Wang, C. W.; Tzeng, H. C.; Wu, C. H.; Chen, Y. C.; Chen, C. P.; Chen, L. C.; Wu, Y. C. J. Am. Chem. Soc. 2006, 128, 3709. (d) Wang, H.; Huff, T. B.; Zweifel, D. A.; He, W.; Low, P. S.; Wei, A.; Cheng, J. X. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 15752. (12) (a) Mohamed, M. B.; Ismail, K. Z.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 9370. (b) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152. (c) Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. Langmuir 1999, 15, 701. (13) (a) Jana, N. R.; Gearheart, L.; Obare, S. O.; Murphy, C. J. Langmuir 2002, 18, 922. (b) Tsung, C. K.; Kou, X.; Shi, Q.; Zhang, J.; Yeung, M. H.; Wang, J.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 5352. (c) Rodrı´guez-Ferna´ndez, J.; Pe´rez-Juste, J.; Mulvaney, P.; Liz-Marza´n, L. M. J. Phys. Chem. B 2005, 109, 14257. (d) Rai, A.; Singh, A.; Ahmad, A.; Sastry, M. Langmuir 2006, 22, 736. (14) (a) Moritz, W.; Wolf, D. Surf. Sci. 1985, 163, L655. (b) Lee, J. I.; Mannstadt, W.; Freeman, A. J. Phys. ReV. B 1999, 59, 1673. (c) Roefof, D.; Foiles, S. M.; Daw, M. S.; Baskes, M. I. Surf. Sci. 1990, 234, 63. (d) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 5417. (e) Zou, S.; Gao, X.; Weaver, M. J. Surf. Sci. 2000, 452, 44. (f) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 772. (g) Gauthier, Y.; Schmid, M.; Hebenstreit, W.; Varga, P. Surf. Sci. 2003, 547, 394. (h) Moritz, W.; Wolf, D. Surf. Sci. 1979, 88, L29.

Sreeprasad et al.

indicating that the reshaping of rods to form spherical particles occurs in a more effective manner. The reaction can be stopped at any stage, resulting in a solution containing nanorods with desired aspect ratio. The reaction, if extended, results in spherical particles. With a careful control over the reaction conditions, it is possible to obtain a multitude of structures varying from truncated rods to fused multibranched structures. All these structures can be stabilized simply by cooling the reaction vessel to room temperature and separating the particles by centrifuging. The in-between structures were imaged using high-resolution transmission electron microscopy (HRTEM). The temperature of the reaction medium and concentration of the reagents were found to be critical factors that determine the shape of the nanoparticles formed. We have isolated the truncated rods and shown clearly that the reaction starts from the tips of a nanorod on specific surfaces in the presence of excess CTAB. The reaction deviates from this path when CTAB is not present in the purified rods, where the reaction happens on the rod body also. This emphasizes the fact that there is preferential reaction in the presence and absence of CTAB. We believe that the truncated rods, with corrugated edges, may show better surface enhanced Raman enhancement compared to the pristine rods. The experimental setup is simple and the reaction parameters are readily tunable. It is possible to obtain differently sized rods from a single starting material by stopping the reaction at different time intervals, but in our case, the range of sizes achievable is restricted by the aspect ratio of the starting material. The mechanism of this process has been established by a series of control experiments. The metal ion induced shape transformation can be expanded to other ions, opening up a prospect for future research. Experimental Details (i) Synthesis of Gold Nanorods. Au nanorods were synthesized by the seed-mediated method reported by Sau and Murphy.6f Briefly, the method uses nanoparticle seeds of 4 nm diameter produced by NaBH4 reduction of HAuCl4. Subsequently, slow reduction of Au3+ ions was carried out using ascorbic acid (AA), in the presence of CTAB and Ag+, resulting in the formation of nanorods having an aspect ratio of 3.44 (with 11 nm diameter). In a typical synthesis, the LSP and TSP appeared at 730 and 518 nm, respectively. The LSP position varied depending on the synthesis. Transmission electron microscopy (TEM) was used to examine the quality of the sample. Optimization of the synthetic procedure yielded high-quality samples, and most of the frames in TEM showed only rods with uniform size. However, spherical particles were seen occasionally. Various experiments were conducted with this material, referred to as R1 (4 × 10-5 M gold), which contained excess CTAB, AA, and ions present in the reaction medium. Experiments were also done with a sample without these impurities. To make this sample, R1 was stored at 4 °C until most of the excess CTAB in the solution settled down. It was filtered and again kept in the refrigerator for 24 h. The solution was filtered and centrifuged at 11 100 rpm for 15 min. The residue was taken and redispersed in triply distilled water. This process ensured effective removal of ionic impurities and excess CTAB and AA in the rod solution. We refer to this sample as R2. The concentration of gold in this solution is less than 4 × 10-5 M, as there was some loss of gold during centrifugation. (ii) Reaction between R1 and CuCl2. In a typical reaction, R1 (4 mL, 4 × 10-5 M gold) was treated with 2 mL of 1 mM CuCl2 in water. Six identical reactions were started simultaneously, and the test tubes were immersed in a water bath maintained at 70 °C. Every 1 h, a test tube was taken out from the water bath and the UV-vis spectrum was recorded with 4 mL of the reaction mixture using a Perkin-Elmer Lambda 25 UV-visible spectrophotometer. A small aliquot of the reaction mixture was dropped on a carboncoated copper grid and dried in ambience. This was observed under

Transformation of Au Nanorods to Nanoparticles a JEOL 3011, 300 kV TEM with a UHR polepiece. This was repeated every hour with different test tubes, and the reaction was found to be complete after 5 h. The reaction was stopped at various time intervals by cooling down the mixture to room temperature, and we found that the nanoparticles formed were stable. (iii) Reaction between R1 and CuCl2 in Argon Atmosphere. The reaction was repeated in argon atmosphere and followed spectrophotometrically. (iv) Reaction between R1 and CuCl2 in the Presence of Na2SO3. The reaction was conducted as described in ii, except that the mixture contained 1 mL of 1 mM Na2SO3 in addition so that the total volume was 7 mL. (v) Reaction between R2 and CuCl2 Solution. The purified nanorods were subjected to reaction as in (ii). (vi) Reaction between R2 and CuCl2 with External Addition of CTAB. In six clean test tubes, 4 mL of R2 (